Relative increases in transcript abundance for the enzymes of ET biosynthesis in three 1-year-old wild-type birch clones, V5818, K1898, and E9702, in response to O3. Clones were subjected to 200 nL L−1 O3 for 8 h, and leaves for RNA isolation were harvested at the times indicated. Hybridization signals of each transcript are indicated as -fold increase of the initial level at 0 h. Black bars indicate the duration of O3 fumigation. A, mRNA levels of BP-ACS1; B, mRNA levels of BP-ACS2; C, mRNA levels of BP-ACO1; and D, mRNA levels of BP-ACO2.